The capability of measuring the flowrates of multiphase fluids in a pipe-without the need to interrupt fluid flow or separate the phases during the measurement process-has been a long-felt need in the chemical and petroleum industries. Applications of such "multiphase flowmeters" are widespread, particularly in the oil business. Because almost all wells produce a mixture of oil, water, and gas, flow measurements of the individual components of the fluid mixture are essential in the efficient production of a reservoir. Today, at the surface, these measurements are made through separators, which are costly and bulky, especially for offshore applications. The availability of a reliable, cheap, compact, and accurate multiphase meter would satisfy an important and long-felt need in oilfield completion and production operations.
There have been many previous attempts to develop multiphase flowmeters. Generally speaking, these prior-art devices attempt to utilize differences in the absorption of X-rays or Gamma-rays by the oil, water, and gas components of the multiphase mixture. As is well-known by persons skilled in the art, such prior-art devices have limited accuracy, particularly in the case of gas fractions above 90%, a common situation in real-world applications.
Examples of prior-art flowmeters using Gamma-ray sources are disclosed in U.S. Pat. Nos. 4,788,852 and 5,025,160, as well as PCT Publication Nos. WO 93/24811 and WO 94/25859, all of which are incorporated herein by reference. Similarly, examples of X-ray-based devices appear in U.S. Pat. Nos. 4,490,609 and 5,689,540, both of which are also incorporated herein by reference.
One feature shared by all of the aforementioned prior-art devices is the use of spectroscopic analysis of the detected of Gamma-rays and X-rays. Specifically, in each of these systems, there exists a detector which measures the energy of each individual photon received, thus creating a spectrographic picture (or histogram) showing the energy distribution of all received photons. Typically, two independent measurements are obtained by setting a "high energy" window (where the Compton Effect dominates) and a "low energy" window (where the Photoelectric Effect dominates). See '852 patent, claim 1 ("A method of measuring the proportions of various components in a crude oil mixture flowing through a pipeline comprising the steps of . . . detecting the gamma rays or x-rays of at least three distinct energy levels passing through a known volume of the mixture to generate three signals . . . "); '160 patent, col. 2, lines 58-65 ("a method for measuring liquid flow velocity of a multiphase flow containing at least two liquid phases, said method comprising the steps of: . . . (ii) measuring at a first location the intensities of gamma-rays at two different gamma-ray energies, . . . "); '24811 PCT, at 9 ("The signals produced by detector 14 in response to the detected gamma-rays are transmitted to a processing computer unit 16 which determines the intensities of the gamma-rays at both energies"); '25859 PCT, at 4, lines 5-7 ("then, by measuring the radiation absorption through a pipline filled with oil, water and gas, at two gamma-ray energy levels . . . "); '609 patent, col. 2, lines, 37-41 ("By spectral analysis of the resultant flux, two measurements related to respectively the lower and the higher energy ranges are produced, from which . . . "); and '540 patent, col. 2, lines 53-55 ("The detector electronics (not shown) bin the detected energy into histograms as a function of energy."). In order to produce such an energy spectrum from the radiation detector, individual X-ray or Gamma-ray photons received by the detector must be separately analyzed.